1. Field of the Invention
The present invention relates to a device for stretching webs of material transversely to their travel direction, which device includes at least one rotary stretcher extending transversely to the travel direction of the web of material.
2. Description of the Related Art
During the processing or treatment of webs of material made of textile, paper, plastic, metal in the form of foils, minerals in the form of nonwovens, and the like, or combinations of these materials, longitudinal wrinkles can develop in the web of material, which interfere with the technological process sequence —for example, during the printing of a web of material—and also adversely affect the straightening and coiling operation. Furthermore, it often happens that a web of material must be cut, and the resulting individual webs must be separated from one another.
To solve these problems, so-called rotary stretchers are used, over which the web of material is guided, and which have the function of stretching the web of material transversely to its travel direction. These rotary stretchers can have various configurations.
Rotary stretchers in the form of thread rollers are known; these rollers are provided over half their length with a right-handed thread and over the other half with a left-handed thread, which are preferably round threads. The web of material running over a thread roller of this type is stretched transversely to its travel direction by the flights, which press the web toward the outside at both ends. The pitch of the threads, which are configured for maximum stretching of the web of material, cannot be changed, which means that the web of material will inevitably slide over the threads and thus be subjected to abrasion.
Another type of stretcher is the so-called rubber cord stretcher, in which rubber cords are embedded in longitudinal grooves in the rollers and thus extend transversely to the web of material. Near the ends of the rollers, these rubber cords are connected to obliquely adjustable disks, which rotate along with the stretchers, so that each of the rubber cords is first stretched over an angle of 180° almost sinusoidally and then relaxed again in the same way over the following angle of 180°. This roller is used in such a way that the web of material rests only on the area of the stretching rubber cords. Due to this configuration and the preset finite number of rubber cords that result from this design, the web of material is not continuously stretched but rather only discontinuously stretched. Moreover, sliding friction occurs in this type of roller, and thus frictional wear between the roller and the rubber cords and between the cords and the web of material also occurs. Due to the friction of the rubber cords in the longitudinal grooves and the molecular friction inside the rubber cords, the required drive torque is so high that it can hardly be applied by the web of material running over the rubber cords. This means that rollers of this type cannot be used without an additional drive adjusted to the speed of the web of material.
Furthermore, it is known that stretchers can be so-called bow rollers or segmented bow rollers with an adjustable radius of curvature. It is this curvature which produces the transverse stretching of the web of material. Bow rollers of this type have a rigid shaft and an elastic sleeve that usually consists of rubber or a cylinder coated with rubber. Due to the high flexing work in the interior of the sleeve, these rollers require a suitable drive, since the necessary torque arising from the coefficient of friction between the rubber and the web of material cannot be applied by the web of material traveling over the rubber. A further disadvantage is that with a high coefficient of friction between the rubber and the web of material, there is also an increase in abrasion between the roller and the web of material longitudinally and transversely to the travel direction.
Finally, it is also known that a roller provided with bristles can be used for the transverse stretching of a web of material. These bristles can damage the web of material, so that rolls of this type cannot be used for sensitive materials such as thin plastic films. The degree of transverse stretching is relatively limited and can be produced for only a limited time due to the gradual weakening of the bristles.